Actually, 900MHz ISM band exists only in ITU Region 2 (ie. Americas), in rest of the world it is usually allocated to GSM and some IIRC aviation related use in

Actually, 900MHz ISM band exists only in ITU Region 2 (ie. Americas), in rest of the world it is usually allocated to GSM and some IIRC aviation related use in the middle of GSM up-down guard band, both are things that you don't want to interfere with, because somebody will care and come after you. 5.8GHz is universally accepted as ISM, but sometimes shared with licensed users (eg. weather radars), who have priority.